1. Field of the Invention
The present invention relates to a copper alloy having both tensile strength and electric conductivity, which is utilized mainly for connectors and sockets, and a method of manufacturing the same.
2. Description of the Related Art
Strips, foils, sheets and rods made of a copper alloy which is press-molded/stamping have been used for general electronic parts. In designing such a copper alloy, the copper alloy is desired to have both tensile strength and electric conductivity, which are contradicting properties. Furthermore, a copper alloy used for electronic parts loaded onto automobiles is desired to have a heat-resistance property in addition to the tensile strength and electric conductivity.
Cu—Zr based alloys are well known as heat resistance lead frame materials. Cu—Zr based alloys increase their heat-resistance property by solid solution of a small amount of Zr into Cu. In Cu—Zr based alloys, Zr supersaturated at a high temperature generates a stabilized compound together with Cu with an aging treatment, thus precipitating. As a result, Cu—Zr based alloys increase their tensile strength (refer to Patent No. 2501275, for example).
However, when Zr is added unnecessarily, a problem arises in that the electric conductivity is decreased. Further, coarse crystals and precipitations which do not contribute to the tensile strength of Cu—Zr based alloys remain, and have a possibility to role as original points of cracks at the time when the Cu—Zr based alloys are press-molded/stamping to make electronic parts such as connectors.
Therefore, the addition of Zr which is considered to be proper has been set roughly from 0.005 to 0.25% in terms of weight ratio (refer to Japanese Patent Laid-Open No. H10(1998)-183274, for example).
However, in such Cu—Zr based alloys, the combination of tensile strength σ (ultimate tensile strength σ) and electric conductivity δ are σ: 300 MPa-δ: 90% IACS (“Precipitation of Alloy” by Nariyasu Kohda, Maruzen, pp 442, FIG. 13.3, issued on Jul. 25, 1972).
Herein, in designing the copper alloy, a dimensionless performance index value M=σ(MPa)*δ(% IACS) is defined to indicate a balance between the tensile strength σ and the electric conductivity δ, which contradict each other. The dimensionless performance index value M of conventional Cu—Zr based alloys remained approximately 270 (300*0.9). Since the conventional Cu—Zr based alloys have a small value of the dimensionless performance index value M, its applicable scope is limited. When a property including both tensile strength σ and electric conductivity δ is required, the conventional Cu—Zr based alloys can not be applicable.
Accordingly, as a copper alloy having a property in that both the tensile strength σ and the electric conductivity δ are high, the copper alloy satisfying an inequality expressed by M>400 is eagerly desired.
On the other hand, in order to increase the value of the dimensionless performance index value M of a Cu—Zr based alloy, the addition of a third element has already been tried.
For example, in Japanese Patent Laid-Open No. H10 (1998)-183274, several copper alloys having the dimensionless performance index value M equal to about 600*0.7=420 have been proposed by adding a small amount of many elements such as Cr and Zn to a Cu—Zr based alloy.
However, exemplification of an alloy achieving the dimensionless performance index value M more than 400 while having a higher tensile strength a (high strength) does not exist. Therefore, there has been a problem in that such a Cu—Zr based alloy is used only for an extremely limited application.
Furthermore, when a Cu—Zr based alloy ends its nominal life as an electronic part and when it is reused as a re-melted raw material, there is a problem in that maintenance of such multiple system alloy scraps is extremely troublesome.